1. Field of the Invention
The present invention relates to a magneto-optical recording medium in which information can be recorded magnetically and from which the magnetic information can be read out making use of a magneto-optic effect.
2. Description of Prior Art
In recent years erasable or rewritable magneto-optical discs have been proposed as a new attractive recording medium. It is expected that the new recording medium will supersede rapidly the conventional unerasable fixed-type optical discs.
The magneto-optical disc proposed as the promising new recording medium is featured by the provision of a perpendicularly magnetized thin film layer formed on a substrate member made of, for example, glass or resin. The magnetic thin film has its axis of easy magnetization orientated in the direction perpendicular to the plane of the disc. To write information in the disc, one uses a laser. By irradiating the magnetic thin film layer with the laser beam the information is thermo-magnetically recorded in the disc. For reproducing the magnetic information stored in the recording medium, one uses the reflected light from the magnetic thin film. The reproduction is attained by detecting the rotation of the plane of polarization (Kerr rotation) of the reflected light caused by magneto-optical Kerr effect.
However, the magneto-optical recording medium as mentioned above has a problem that the S/N ratio (Signal-to-Noise ratio) attainable in reproducing information is too low to be satisfactory. This problem of unsatisfactory S/N ratio is mainly attributable to the following facts:
The magnetic thin film now being used as the perpendicularly magnetized thin layer of the recording disc is able to produce only a very small Kerr rotation angle which is in the order of about 0.1.degree. (degree). In addition, the power of light beam usable for the reproduction must be limited so as not be have the stored magnetic information lost by the reproducing light beam.
As a solution to the problem it has already been proposed to apply a film of dielectric material on the magnetic thin film of the recording disc. The dielectric film has an effect to reduce the reflection factor of the recording medium and thereby apparently increase the Kerr rotation angle, which leads to improvement in the S/N ratio. This solution is disclosed, for example, in Japanese Patent Application laid-open No. 156,943/1981. An example of the magneto-optical recording medium designed according to the solution is shown in FIG. 1 of which a description will be made hereinafter to assist in better understanding of the present invention.
Referring to FIG. 1, the prior art magneto-optical recording medium comprises a substrate 1, a magnetic thin film layer 2 and a reflection factor regulating layer 3. The magnetic thin film layer 2 is formed on the substrate 1 employing a suitable known technique such as vacuum deposition or spattering. The top regulating layer 3 on the magnetic thin film 2 is formed of a dielectric substance. Reading of the stored information for reproduction is carried out by projecting a reading light A on the recording medium from above and detecting the Kerr rotation of the reflected light from the recording medium. FIG. 2 illustrates the state of polarization of the reflected light. In FIG. 2, x-axis is in the polarization direction of the incident light and y-axis orthogonal to x-axis is in the Kerr effect polarization direction. As shown in FIG. 2, the reading light A is polarized into an elliptically polarized light having a component in the Kerr effect polarization direction (y-axis). The major axis of the ellipse and the x-axis form an angle .theta.k which is called Kerr rotation angle. The Kerr rotation angle .theta.k is given by: ##EQU1## wherein, rx is Fresnel reflection factor in the incident
light polarization direction of the reflected light; PA1 ry is Kerr reflection factor of the orthogonal component produced by Kerr effect; and PA1 .delta. is phase difference between rx and ry.
To detect the reflected light there may be used a photo detector having a current multiplying function such as avalanche photo diode (A P D) or a photo detector having no current multiplying function such as P I N photo diode. In the former case, the S/N ratio of the reproduced information signal is proportional to .sqroot.R.multidot..theta.k wherein R is the power reflection factor of the recording medium. Therefore, from the above equation (1) and .sqroot.R=.vertline.rx.vertline., the S/N ratio in reproduction is given by: EQU (S/N).alpha..vertline.ry.vertline. cos .delta. . . . (2)
Since .vertline.ry.vertline..alpha.(1-R), it will be understood that the S/N ratio in reproduction can be improved by decreasing the reflection factor of the recording medium.
In the latter case where a photo detector having no current multiplying function such as PIN photo diode is used, the relationship between reflection factor and S/N ratio is not so simple as described above for the former case. However, in the latter case also it is known that the S/N ratio in reproduction can be improved by decreasing the reflection factor of the recording medium and adjusting it to the optimum value.
In the example of the prior art shown in FIG. 1, the regulating layer 3 is formed on the magnetic thin film layer 2 by vacuum deposition of dielectric substance having a high refractive index such as ZnS or TiO.sub.2 in order to reduce the reflection factor of the recording medium thereby improving the S/N ratio in reproduction of information. Obviously a satisfactory S/N ratio can be obtained only when the reflection factor is sufficiently decreased by the provision of the dielectric regulating layer 3. In order to attain it, the layer 3 of high refractive substance must be formed as a high density film layer. This means that when the dielectric regulating layer 3 is formed, the whole of magnetic thin film layer 2 and substrate 1 must be heated to a high temperature. However, as well-known to those skilled in the art, the magnetic characteristics of the magnetic film 2 are generally deteriorated by such high temperature heating. Therefore, the use of such a high temperature is undesirable. Especially, in the case of amorphous material such as GdTbFe, such high temperature heating results in crystallization of the amorphous material by which the magnetic property of the material is greatly changed undesirably. For this reason, it has been deemed undesirable to sufficiently heat the substrate when the dielectric regulating layer is formed. Therefore, it has been impossible to obtain a desired high refractive index for the regulating layer 3. Consequently the magneto-optical recording medium according to the prior art as shown in FIG. 1 has only a limited improvement in S/N ratio although it is provided with a reflection factor regulating layer. This is an important drawback of the above-mentioned solution.
As another solution to the above problem it has already been proposed to form a very thin magnetic film on a film layer of a metal such as Cu or Al. The film thickness permissible for the magnetic film is, at the largest, about the penetration depth of the reading light. This second solution is based on the finding that the Kerr rotation angle can be increased making use of Faraday effect by the multiple reflection within the magnetic thin film. This solution is disclosed, for example, in British laid-open Patent No. 2,094,540. FIG. 3 shows an example of the prior art magneto-optical recording medium designed according to the second solution.
Referring to FIG. 3, the recording medium comprises a substrate 11, a metal film layer 12 formed on the substrate 1 by vacuum deposition or other suitable known technique, and a magnetic thin film layer 13 formed on the metal film layer 12 by vacuum deposition or spattering. Again, A denotes a reading light which is projected on the recording medium from above. To reproduce the information previously stored in the recording medium the Kerr rotation of the reflected light is detected in the same manner as above. In this prior art recording medium, the light A transmitted through the top magnetic thin film layer 13 is reflected on the interface between the magnetic layer 13 and the metal layer 12. Further, the reflected light is multiple-reflected in the magnetic film layer 13 so that the Kerr rotation angle of the reflected light can be increased by Faraday effect and therefore the S/N ratio in reproduction can be improved accordingly.
However, the prior art recording medium as shown in FIG. 3 also has some drawbacks.
Firstly the reflecting power of the interface between magnetic layer 13 and metal layer 12 is not sufficiently high for producing a desired effect to adequately increase the Kerr rotation. Therefore, improvement in S/N ratio attainable by it is not so great.
Secondly, the metal film layer 12 undesirably acts as a heat sink. This reduces the recording sensitiveness for thermo-magnetic writing of information in the recording medium. Especially, the writing property of the recording medium for high frequency signals is greatly deteriorated by the metal layer 12.
Because of these drawbacks of the prior art recording medium a further improvement in the S/N ratio is highly desired.